Generally, shoes consist of an insole, i.e. the interior bottom of a shoe, an outsole which contacts the ground, and in some cases a midsole between the outsole and the insole. The purpose of a midsole is often to act as a shock absorber to absorb the impact of walking and provide extra cushion, and for this reason materials such as EVA and polyurethane are often used.
Orthotic inserts, also called arch supports, are inserts placed into a shoe either on top of or in place of the insole. Orthotics are used to correct foot alignment and side-to-side movement during walking and thereby decrease pain, not only in the foot, but also in other parts of the body such as the knee, hip and lower back. They can also increase stability in an unstable joint and prevent a deformed foot from developing additional problems. Rigid orthotic devices are generally made by casting a mold, and can be made from materials such as plastic or carbon fiber.
Carbon fiber and steel plates inserted into a shoe are also known for use in stiffening the sole of the shoe. Such inserts can provide additional comfort for individuals with arthritis.